What is a Transmission Color pass?
Answer is in your first two lines. Transmission pass is nothing else than colors of transmissions rays. Imagine diffuse color pass. It's color only. No other influence like shadows and glossy. Transmission pass is exactly the same, but color of transmission rays. Make your cube red (in glass shader) and you get red color in transmission pass. 

PS: Does not make sense write the same few times in documentation

PS2: maybe you try find something that not exist :-)